Hi guys,
I have a '66 396-325 HP. Original Harrison-thermostat 180 deg. Aftermarket hoses. Problem- when driving the car for even a short distance the overflow goes wild upon returning home coolant all over the driveway). I have a warning light instead of guages so I don't really know the water-coolant temp. Thermostat has been tested in hot water and funtions perfectly at 180 deg. I recently purchased the car and I'm concerned there may be a blockage somewhere in the system. I did check flow through the top of radiator with open thermostat and it appears to flow strongly at least at the top of the radiator. Any ideas? How to backflush? Do I need a radiator shop to diagnose? Scary to have anyone work on this classic! I have had very bad success with aftermarket cooling system cleansers. One ate through my head gasket! and ruined the engine (acid based)
Your thoughts?
Thanks,
DWR

is your water level above the fill line?
if so thats why its puking out water.

Every chevelle I've had holds pressure in the cooling system after getting up to operating temp. and shutting it down. Overfilling the radiator or having a bad radiator cap will lend itself to the coolant being pushed out by the pressure, or coolant expansion.

Donnie mentioned the problem with filling above the fill line, located just below the fill opening on a harrison radiator. A weak cap will also allow similar symptoms.

Is your radiator pressure cap (15lb) working correctly? Are you running with any coolant or just water? Any good shop should be able to back flush the system and pressure test it. There are back flush kits for the do it yourselfer if you don't mind the extra fitting in the heater hose - but then you can always replace the hose with you're finished.

Donnie1 is correct, if the water level is too high to begin with there's no room for expansion when it gets warm and it'll seek an exit path. ;)

You can have your original Harrison by a good radiator shop and have your unit built to a four core and with more fins than stock. I did that.

Is it over heating or just pushing the expanded water out? You could stick an over flow tank on it to catch the blow off. Another possibility is that the motor is out of tune timing wise or your carburator is running lean.

More than likely, if you are not overfilling the radiator it's a weak cap. Have your cap pressure checked or just get a new one. Make sure you're not overfilling it.
When you stop, keep it at a fast idle for a minute before shutting it off. If you have been running at highway speed there is a lot of heat built up in the block that needs to be cooled off before shutting down.
It's a good idea to run for about a minute or so when pulling into a rest stop from the freeway. Keeps the system from puking after a long run.

To play it safe & save some dough, drain out the coolant completely and do your own reverse backflush. The Prestone products are pretty safe; they're not acidic. Use their Super Cleaner and Super Flush products. And follow the directions! I've always had very good results without any damage. After you've done this a few times, fill up with distilled water & an anti-corrosion lubricant. Run the engine on the driveway and see if your problem is solved.
btw, you may need to change your engine thermostat as well.